The present invention relates to a Mode select (Mode-S) air traffic control radar beacon system (ATCRBS); and more particularly to an enhanced performance Mode S interrogator subsystem. For efficient management of high traffic environments it is essential that an ATCRBS accurately identify and locate potentially closely spaced aircraft. In general Mode S ATCRBS selectively identifies and interrogates individual aircraft using known half angle phase-comparison monopulse azimuth determination and a multi-pattern antenna 10 such as shown in FIG. 1. The pattern includes three patterns, a sum pattern 15, difference patterns 20, and an omnipattern 25. As is evident from FIG. 1, the sum pattern 15 is equivalent to a single main beam of a non-monopulse system. When a target is located at an angle .theta. off-bore site, the sum and difference patterns detect returns with differing amplitudes and phases. By applying known coherent phase comparison techniques, the angle .theta. can be accurately determined using a ratio of the sum and difference signals. However, to accurately determine the angle .theta., sum and difference channels in a receiver must be carefully matched in terms of, for example, phase response, group delay and frequency response. This places strict requirements on these channels, and in particular, filters used within the interrogator receiver.